1. Field of the Invention
This invention relates to a liquid crystal display device making use of the change of orientation of liquid crystal molecules which are aligned to a proper direction in advance by applying or changing the intensity of an electric field, a magnetic field, heat or the like, and more particularly to an alignment film of the display device.
2. Brief Description of the Prior Art
The liquid crystal display device is basically comprised of two glass substrates disposed in parallel with each other at a given distance, transparent electroconductive film strips being selectively disposed on the opposing surfaces of these two substrates, an alignment film for aligning liquid crystal molecules in a definite direction which completely covers each of said substrate surfaces and the electroconductive strips placed thereon, and a liquid crystal filling the space between the alignment films, the side faces of the device being tightly sealed lest any leakage of the liquid crystal should occur. At times it is further provided with a polarization plate on the outer surface of one of the substrates.
(A) Dynamic scattering type, (B) electric field induced birefrengence effect type, (C) guest-host effect type, (D) phase transition guest-host effect type, etc. are the well-known types of display system employing a device basically of the above-described structure, and (D) type system is the most practical one.
If the above-mentioned (D) type system is employed, (i) coloured display as well as black-and-white display can be made and (ii) the production cost can be reduced by the simplification of element structure. For this reason it is very likely that these systems will eventually replace the hitherto widely used twist-nematic electric field effect type display system employing horizontally aligned films. And in the use of system with vertical alignment of liquid crystal molecules, improved display characteristics can be achieved.
The following alignment films (a)-(e) have hitherto been used as films for aligning liquid crystal molecules vertically:
(a) Vapour-deposited films of rare earth metal oxides, etc.
(b) Films of silicon-containing metal oxides, etc.
(c) Film consisting of a surface active agent containing a long alkyl chain or fluoroalkyl chain group.
(d) Film of a complex compound of a carboxylic acid containing a long alkyl chain or fluoroalkyl chain group.
(e) Film of a silane compound containing a long alkyl chain or fluoroalkyl chain group.
The above alignment films, however, have the following disadvantages.
(i) Both alignment films (a) and (b) show different aligning ability, depending upon the kinds of liquid crystals, and no alignment is obtained at all, when some kinds of liquid crystal are used. Besides, their film surface is active so that they are likely to be contaminated with an organic compound gas, when a cell is assembled. In the case of the guesthost type display device, the film selectively adsorbs a dichroic dye added to the liquid crystal, resulting in poor alignment.
Moreover, a liquid display device will be expensive, because of use of an expensive vapor deposition apparatus and of many working steps involved.
(ii) Films (c) and (d) tend to lose their molecule aligning properties because of their poor heat-resisting property when they are heated during cell assembly. It is difficult to obtain a thick coat from these materials, and besides the coat is liable to include electroconductive substances therein and is easily released from the substrate. Thus, the life is poor.
(iii) Film (e) has a problem in coating, and a smooth film cannot be obtained. Thus, the alignment is poor. Furthermore, it is difficult to obtain a thick film and the life is poor.
Moreover, the pattern of the transparent electroconductive strips formed on the substrates appear too clearly as if embossed, because the film (e) has a refractive index of less then 1.6.